Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. A transmission speed ratio is the ratio of input-shaft speed to output-shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Most transmissions are equipped with a torque converter or other type of launch device. When the vehicle is stationary or moving very slowly, the gearbox input speed is less than the minimum operating speed of the engine. A launch device transmits torque from the engine to the gearbox input while permitting the engine to rotate at an acceptable speed. A torque converter includes an impeller driven by the engine and a turbine driving the gearbox. Torque is transferred from the impeller to the turbine hydrodynamically.
Automatic transmissions include several modes including PARK, REVERSE, NEUTRAL, and DRIVE. The driver selects between the different modes using an input, e.g., a shifter, located in the passenger cabin. The transmission includes clutches and brakes operable to create one or more gear ratios associated with one or more of the modes. For example, the transmission may include five forward-drive gear ratios associated with DRIVE, a single reverse gear ratio associated with REVERSE, and one or more neutral states associated with NEUTRAL and PARK.
Some transmissions are capable of having multiple neutral states in which different combinations of clutches and/or brakes are locked while still achieving NEUTRAL. U.S. Pat. No. 8,162,796 discloses this type of transmission.